feat(decider): Use streams instead of options [LNG-277]

[InversionSpaces]

In the next aqua release options will become immutable. Streams should be used instead.

[linear]

LNG-277 Prohibit mutating options

Right now it is possible to define options just like streams and mutate them:

maybe_value: ?string
...
maybe_value <<- value

It is documented in docs and used a lot in aqua code.

But option implicitly canonicalized on return, making them immutable:

func create_option() -> ?string:
  option: ?string
  option <<- "value"
  <- option

o <- create_option()
o <<- "new value" -- Error here

Also if a function accepts an option, it is prohibited to mutate it:

func use_option(option: ?string):
  option <<- "default value" -- Error here

Moreover, there is no guarantee that option will actually contain at most one element (bc under the hood it is an array or a stream), except that for immutable options compiler will reject non-zero index access:

func use_option():
  option: ?string
  a = option[10] -- OK, which is strange

func use_option(option: ?string):
  a = option[10] -- Error here

Also, streams are implicitly convertable (canonicalized) to options, so popular usage pattern is supported by them:

func create_option() -> ?string:
  option: *string
  option <<- "value"
  <- option

Everything described above makes options inconsistent data type, complicating compiler and aqua code. This is why it seems better to:

• prohibit creating options with o: ?type syntax
• prohibit mutating options everywhere

[kmd-fl]

Now the code looks more confusing :c